Devices for measuring the fuel level in motor vehicles typically consist of a float device in the fuel tank which includes a variable impedance element whose impedance varies in proportion to the fuel level in the tank. Typically, this impedance is read by a coil-type fuel gage mounted in the dashboard. Conventional fuel level measuring devices produce a continuous current flow through the variable impedance element of the float device in order to maintain a steady fuel reading by the gage. Conventional coil-type fuel gages in use today are nonlinear devices. That is, a nonlinear change in gage current must be supplied to produce a linear change in gage needle deflection.
The increased use of alcohol based fuel has created difficulties in using traditional means for measuring fuel levels. The continuous current flowing through the float device element causes electrolysis due to the alcohol in these fuels, resulting in the eventual corrosion of the float device. It has been proposed to send current through the float device at periodic intervals to reduce the corrosion caused by electrolysis. In this regard, some have experimented with using digital microprocessor devices to read the float data through an analog-to-digital converter. However, this purely digital approach requires complex data conversion routines to compensate for the nonlinearity of the coil gage mechanism. Since these complex data conversion routines require expensive analog-to-digital converters and expensive microprocessors, this approach is too expensive and not practical for most mass produced automotive vehicles.
Thus it is an object of the present invention to provide an inexpensive, practical and yet accurate apparatus for measuring fuel level using a pulsed energy source to retard electrolysis in the float device. It is a further object to provide an inexpensive analog apparatus which matches the nonlinear needle deflection versus current characteristics of the coil gage over the entire operating range, thereby eliminating the need for expensive analog-to-digital converters and complex data conversion routines.
To realize the above objects, there is provided, in accordance with one aspect of the present invention, a circuit for interconnecting a variable impedance fuel level sender of a fuel tank with a fuel gage of the type having nonlinear characteristics. This circuit comprises: a synchronizing circuit; a pulsed energy source controlled by the synchronizing circuit and coupled to the sender for periodically energizing the sender; an impedance matching circuit having a Thevenin impedance substantially equal to the Thevenin impedance of the gage and coupled to the sender for sensing a condition indicative of the level of fuel in the tank; a sample and hold circuit coupled to the impedance matching circuit and to the gage, controlled by the synchronizing circuit for sampling the condition sensed by the impedance matching circuit and supplying a signal for causing the gage to display the level of fuel.
The synchronizing circuit is a low cost microprocessor or microcontroller capable of sending a sequence of signals. The pulsed energy source is capable of supplying a voltage level in proportion to the input ignition voltage. In the presently preferred embodiment the impedance matching circuit is composed of two resistors which are coupled to have an impedance substantially equal to that of the gage. One of the resistors is further connected in parallel with the sender and in series with the second resistor. The sample and hold circuit is activated by the synchronizing circuit after the pulsed energy source has activated the sender.
These and other objects, advantages and aspects of the present invention may be further understood by referring to the detailed description, accompanying figures and appended claims.